Planted Tank Guru
Join Date: Jan 2010
Location: Contra Costa CA
Rocks that are outside, freshly 'harvested' may present some problems, but with a little care many rock are useful.
Select the location away from pollutants.
Stay away from roads. Oil and other fluids dripping from vehicles, and salt (snow) can linger on the rocks.
Stay away from agricultural fields and gardens where pesticides are used.
Get permission from the land owner.
Do not collect in parks of any kind.
If the rock looks like it is already crumbling, it will do that in the tank, too.
Rocks that are obviously metal bearing are not good. Copper usually looks somewhat blue-green, iron will usually rust, iron pyrite will look shiny, sort of like gold.
I have used rocks with a small trace of iron in a large tank and not had issues. (Small amount = a few spots about the size of a dime in 125 gallon tank)
Rocks may be limestone or some variation. Put a little acid on them. If they bubble, they are probably not good in a soft water tank. Acids to try:
Vinegar, API Nitrate test bottle #1, Muriatic Acid (swimming pool supply) BE CAREFUL Acids can harm you.
Another way to test rocks is to put them in a bucket of water (especially used aquarium water, but tap water is fine) and test before and every few days while the rocks soak. GH, KH, pH, TDS. If there are tiny changes it is probably OK If there are larger changes then this suggests the rocks will affect the aquarium water.
If the rocks come from a stream or lake rinse well in bleach, then rinse with dechlor in the water. If you still smell chlorine put the rocks in the sun and turn them around every few hours. Chlorine will evaporate.